console: Implement flush() function

On certain places it is required to flush output print buffers to ensure
that text strings were sent to console or serial devices. For example when
printing message that U-Boot is going to boot kernel or when U-Boot is
going to change baudrate of terminal device.

Therefore introduce a new flush() and fflush() functions into console code.
These functions will call .flush callback of associated stdio_dev device.

As this function may increase U-Boot side, allow to compile U-Boot without
this function. For this purpose there is a new config CONSOLE_FLUSH_SUPPORT
which is enabled by default and can be disabled. It is a good idea to have
this option enabled for all boards which have enough space for it.

When option is disabled when U-Boot defines just empty static inline
function fflush() to avoid ifdefs in other code.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>